1. Field of the Invention
The present invention relates generally to spray cooling thermal management systems and more specifically it relates to a spray cooling system that has improved thermal performance through the use of etched open mircochannels.
2. Description of the Related Art
Liquid cooling is well known in the art of cooling electronics. As air cooling heat sinks continue to be pushed to new performance levels, so has their cost, complexity, and weight. For some time, liquid cooling solutions have been developed and tested, but other than specialty applications they have yet to gain widespread commercial adoption. As computer power consumptions continue to increase, liquid cooling will provide significant advantages to computer manufacturers which will force its use. The present invention provides significant advantages over both air cooling and prior art liquid cooling solutions.
Liquid cooling technologies use a cooling fluid for removing heat from an electronic component. Liquids can hold and transfer heat at a rate many times that of air. Single-phase liquid cooling systems place a pure liquid in thermal contact with the component to be cooled. With these systems, the cooling fluid absorbs heat as sensible energy. Other liquid cooling systems, such as spray cooling, are two-phase processes. In these systems, heat is absorbed by the cooling fluid as latent energy gains. Two-phase cooling, or commonly referred to as evaporative cooling, provides the ability to deliver more efficient, more compact and higher performing liquid cooling systems than single-phase systems.
An exemplary two-phase cooling method is spray cooling. Spray cooling uses a pump for supplying fluid to one or more nozzles that transform the coolant supply into droplets. These droplets impinge the surface of the component to be cooled and can create a thin coolant film. Energy is transferred from the surface of the component to the thin-film. Because the fluid is dispensed at or near its saturation point, the absorbed heat causes the thin-film to turn to vapor. This vapor is then condensed, often by means of a heat exchanger, or condenser, and returned to the pump.
Significant efforts have been expended in the development and optimization of spray cooling. A doctorial dissertation to Tilton entitled “Spray Cooling” (1989), available through the University of Kentucky library system, describes how optimization of spray cooling system parameters, such as droplet size, distribution, and momentum can create a thin coolant film capable of absorbing high heat fluxes. As described by the Tilton dissertation, atomization plays a key role in the development of a thin coolant film capable of absorbing very high heat fluxes, such as a coolant film capable of absorbing a heat flux over 800 watts per square centimeter. Research and development by Isothermal Systems Research (ISR) has shown spray cooling to be capable of absorbing heat fluxes in the range of several thousands watts per square centimeter.
In addition to the system parameters described by the Tilton dissertation, U.S. Pat. No. 5,220,804 provides a method of increasing a spray cooling system's ability to remove heat. The '804 patent describes a method of managing system vapor that further thins the coolant film which increases evaporation, improves convective heat transfer, and liquid and vapor reclaim.
Historically, most electronic cooling solutions have provided wide area treatment of the cooling surface. Electronic components are rated to a total wattage that is spread by an aluminum, copper, or diamond heat spreader to the cooling fluid (may be air or liquid). In some applications, this wide-area average heat flux treatment of the cooling surface only marginally takes advantage of the benefits of liquid cooling over air cooling.
Electronic components create varying amounts of heat across their surfaces and a varying amount of heat as a function of time. Today's microprocessors, for example, may be constructed on a silicon die roughly 1 cm by 1 cm. The die may have multiple zones for different functions. Such zones may be for inputs and outputs (I/Os), level 1 cache, level 2 cache, and the core. The core may be roughly 0.5 cm by 0.5 cm and is where the main computer processing takes place. Although the core may only be 25% of the total area of the die, it creates almost the entire heat generation by the chip and may be considered a chip “hotspot”. Wherein a chip might be rated for an average heat load of 110 watts, with an average heat flux of 110 wafts per centimeter squared, the core may generate 100 watts of that heat and have a heat flux of 400 watts per centimeter squared. This non-uniform heat flux distribution poses a significant challenge to the cooling system as it is desirable to keep the entire chip at nearly the same operating temperature. Cooling systems that rely on heat spreading may not provide this ability as they rely on conduction spreading, resulting in significant temperature gradients across the chip.
One method of cooling the core of a computer chip is to divide the chip into thermal zones and to cool each of the chip's zones at a different rate. U.S. Pat. No. 6,443,323, describes a method of variably cooling a computer component through the use of incremental sprayers. The incremental sprayers deposit fluid onto each zone at a mass flow rate necessary for complete phase change. Drops are ejected from an orifice in serial. Although this method improves the efficiency of the system, that is in attaining complete phase change of all dispensed fluid, the incremental dispensing method does not provide dispensing characteristics necessary to create high heat flux thin-film evaporative cooling and high performance cooling of hotspots. A paper by Don Tilton and Jay Ambrose (1989), entitled “Closed-System, High-Flux Evaporative Spray Cooling”, describes the early development and analysis of incremental sprayers and predicts a maximum heat flux capability of around 300 watts per centimeter squared using water. An ASME paper published by Bash, Patel, and Sharma entitled “Inkjet Assisted Spray Cooling of Electronics” (2003), describes an inkjet dispensing system with a critical heat flux of around 270 wafts per centimeter squared using water.
Another method of cooling the core is described by U.S. Pat. No. 6,650,542. Although this method directs and controls the single phase fluid over a chip hotspot, the disclosed method is not a phase change process and thus not capable of high heat flux thin-film evaporative cooling.
Yet another method of cooling the core is two-phase microchannels, such as described by U.S. Pat. No. 4,450,472. Although this method does not use spray cooling, the design does provide the ability to remove heat in the range of 400–1000 watts per square centimeter using water. The system discloses a method of placing a very small microchannel array on an electronic component. Although microchannel cooling methods may effectively lower the temperature of the core, due to large pressure drops and resulting size limitations they do not efficiently address the needs of the non-hotspot areas of the die.
For the foregoing reasons, there is a need for a liquid cooling solution that effectively cools the one or more hotspots of a computing component. Thus, there is a need for a localized cooling solution capable of absorbing large heat fluxes. Also, the high heat flux cooling system must efficiently and reliably cool the other non-high heat flux areas of the chip. The resulting cooling solution would allow significant improvements in processor performance.